Method of removing organic flux using peroxide composition

ABSTRACT

A composition for removing organic contaminants, such a flux residues, from a solid substrate comprises: (a) hydrogen peroxide in the amount of about 3 to 5 percent by weight of the composition; (b) an alkaline compound in sufficient amount to provide a pH of at least 10.5 in the composition; (c) about 0.1 to 0.3 percent by weight of a chosen wetting agent which is unreactive with the hydrogen peroxide and the alkaline compound; and (d) purified water as the balance of the composition. Optionally, the composition may further comprise about 0.5 to 2.0 percent by weight of a chosen metal protective agent. 
     The solid substrate having organic contaminants thereon is exposed to the above noted composition whereby the organic contaminants are removed from the substrate and are converted into non toxic and non-hazardous products. Thus, negative environmental impact is avoided by the present process. In an alternative embodiment, the organic contaminant removal is further enhanced by exposing the composition and the organic contaminants on the substrate to ultraviolet radiation.

This is a division of application Ser. No. 07/453,446, filed Dec. 20,1989 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a composition and method forremoving organic contaminants from solid substrates. More particularly,the present invention relates to a hydrogen peroxide composition forremoving organic contaminants such as solder flux; and to a method ofusing this composition.

2. Description of the Background Art

In the fabrication of printed circuit boards, integrated circuits, andvarious electronic components and devices, solder is used to joinvarious component metal parts together. The solder itself comprises analloy, such as of zinc and copper or of tin and lead. Prior toapplication of the solder to the metal surface, the surface is treatedwith a fluxing agent to remove oxides and other contaminants which mightinterfere with the metal bonding, and to prevent reformation thereofduring the solder process. These fluxing agents are typically organicmaterials such as natural rosin extracted from pine tree sap, organicacids such as carboxylic acid, hydrazines, amines and amides, orinorganic materials such as inorganic acids or salts. The most commonlyused fluxing agent is acid rosin flux. The term "rosin flux" is usedherein to mean a flux material which comprises rosin, i.e., the resinafter distilling turpentine from the exudation of species of pine, andcontaining abietic acid and its anhydride. Typically, a small amount ofanother acid is added to rosin flux to activate it and such compositionsare referred to as "acid rosin flux" or "activated rosin flux." Afterthe solder operation has been completed, excess flux as well as fluxresidues remain on the soldered surface, and these residues areresinous, waxy and conductive. These flux residues and excess flux mustbe removed prior to subsequent processing steps in order to preventreaction thereof with the bonded part, leading to corrosion andresultant electrical insulation losses.

Defluxing solvents which are widely used at present are halogenatedhydrocarbons such as 1,1,1-trichloroethane and Freon (a tradename of E.I. DuPont for polyhalogenated hydrocarbons including chlorine andfluorine). While these organic materials are effective defluxingsolvents, they have the serious disadvantage that they have a negativeimpact on the environment due to air pollution and ozone depletion. Infact, recent environmental legislation mandates that these materials bebanned or their production severely restricted in the next few years.When these materials are used, even in small quantities, expensivemanagement systems for transport, storage, use, and disposal andenvironmental protection equipment must be used to prevent air and waterdischarges. In addition, waste solvents require energy intensiveregeneration operations for these materials.

While numerous aqueous cleaners are available which are viabledegreasing solvents, none of these has been found to be effective as anelectronic component defluxing solvent. In addition, the resultingorganic laden aqueous solvents require further processing beforedisposal.

Thus, an urgent need exists in the electronics industry for a solventwhich effectively removes organic flux residues while at the same timeavoiding any negative environmental impact. Such a solvent would also bedesirable for removing other organic materials from other substrates.

SUMMARY OF THE INVENTION

The general purpose of the present invention is to provide a new andimproved composition and method for removing organic contaminants from achosen solid substrate while avoiding negative environmental impact.This composition and method possess the advantages of the above priorart processes while overcoming their significant disadvantages.

The above general purpose of this invention is accomplished by firstproviding a composition comprising: (a) hydrogen peroxide in the amountof about 3 to 5 percent by weight of said composition; (b) an alkalinecompound in sufficient amount to provide a pH of at least 10.5 in saidcomposition; (c) a chosen wetting agent in the amount of about 0.1 to0.3 percent by weight of said composition, said wetting agent beingunreactive with said hydrogen peroxide and said alkaline compound; and(d) purified water as the balance of said composition.

Optionally, the composition may further comprise a chosen metalprotective agent in the amount of about 0.5 to 2.0 percent by weight ofsaid composition, said metal protective agent being unreactive with saidhydrogen peroxide and said alkaline compound.

The solid substrate having organic contaminants thereon is exposed tothe above noted composition whereby the organic contaminants are removedfrom the substrate and are converted into non-toxic and non hazardousproducts. Thus, negative environmental impact is avoided by the presentprocess. In an alternative embodiment of the present invention, theorganic contaminant removal is further enhanced by exposing thecomposition and the organic contaminants on the substrate to ultravioletradiation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The composition of the present invention is effective for removingorganic contaminants from a solid substrate while at the same timeavoiding undesirable impact on the environment. The composition of thepresent invention is especially useful for removing flux residues whichcontaminate surfaces after a soldering operation. These flux residuestypically comprise resinous, waxy contaminants which are the breakdownproducts from the soldering operation. The substrates which are solderedcomprise, for example, printed wiring boards, integrated circuits,electronic components, electronic devices, electronic connectors, orelectronic cables. In accordance with the present invention, theby-products, such as carbon dioxide, nitrogen, and water, which areformed are non-hazardous and can be removed without having a negativeenvironmental impact.

The composition in accordance with the present invention comprises: (a)hydrogen peroxide (H₂ O₂) in the amount of about 3 to 5 percent byweight; (b) an alkaline compound in a sufficient amount to provide a pHof 10.5 or higher in the composition; (c) a chosen wetting agent whichis unreactive with hydrogen peroxide and the alkaline compound and whichis present in the amount of about 0.1 to 0.3 percent by weight of thecomposition; and (d) purified water as the balance of the composition.Optionally, if the substrate comprises metal, a metal-protective agentis added to protect the metal surface from attack by the peroxide andalkali. The metal protective agent is unreactive with the hydrogenperoxide and the alkaline compound and is present in the amount of about0.5 to 2.0 percent by weight of the composition.

The hydrogen peroxide acts as a solvent, emulsifier, and oxidant. Whilenot limiting the present invention to a particular theory of operation,it is believed that the hydrogen peroxide has the following effect.After being immersed in the present composition, a flux-contaminatedsubstrate invokes selective adsorption of hydrogen peroxide into theflux film and rapid decomposition of the inorganic peroxide. Theselective adsorption of hydrogen peroxide by the rosin based contaminantfilms is due to the ether like solubility chemistry of hydrogenperoxide. Following adsorption, hydrogen peroxide rapidly decomposesinto water and oxygen gas. The oxygen emulsifies the resinouscontaminant, increasing surface area and solvency. This adsorption andemulsification process is observed as spontaneous foaming on thecontaminated surface. The scrubbing foam enhances the cleaning activityof the solvent, dramatically improving penetration of solvent intolow-tolerance spacings. After the organic flux contaminants have beenremoved from the substrate, they are oxidized by the action of nascentor atomic oxygen which is formed by the spontaneous decomposition ofhydrogen peroxide, and are decomposed into carbon dioxide, nitrogen, andwater. This effect is evidenced by a change in color in the compositionfrom amber when it contains dissolved flux, to clear. The contaminantsmay be dissolved or suspended material.

The effectiveness of the composition of the present invention isbelieved to be due to a synergistic relationship between pH, peroxidechemistry and the wetting agent. It was found that peroxide basedformulations with pH's of 2 (acidic) and 7 (neutral) did not effectivelyremove contaminants. In these cases, the flux contaminants tended to geland discolor on the substrate surfaces. Alkaline formulations having apH of greater than 10.5 were considerably more effective at solubilizingthe flux residues than acid or neutral solutions. (This is probably dueto ionization of flux acids by basic solutions, forming primativesoaps.) Thus, the alkaline compound is added to the present compositionto provide a pH of at least 10.5, preferably within the range of 10.5 to11.5. The alkaline compound may be, for example, sodium hydroxide orpotassium hydroxide, with sodium hydroxide being most preferred. Ifsodium hydroxide is used, it is present in the amount of about 0.2 to0.5 percent by weight of the composition.

The wetting agent must be chosen to be compatible with the othercomponents in the present composition. Conventional wetting agents suchas sodium alkylaryl sulfonate and other organic synthetic detergentsdecompose rapidly in the presence of strong alkaline oxidizing/bleaching solutions such as the composition of the present invention.This results in rapid decomposition of the hydrogen peroxide solvent andexcessive foaming in the solution. A preferred wetting agent for use inthe present composition comprises sodium 2-ethylhexyl sulfate (obtainedfrom Niacet Co., Niagara Falls, N.Y.). It has good solubility,stability, and penetrating action in near-boiling alkaline and acidsolutions, and is one of the few anionic surfactants stable inconcentrated bleaching solutions. Other suitable wetting agents forpractising the present invention may comprise sodium metasilicate orshort chain branched surfactants. The wetting agent is used in thepresent invention in the amount of about 0.1 to 0.3 percent by weight ofthe composition. If the substrate comprises a metal, such as in aprinted wiring board, the metal surfaces must be protected from attackby the peroxide and alkali in the present composition. The particularmetal protective agent used depends on the specific metal beingprotected. For example, calcium and phosphorous compounds are used asprotective agents for copper. The selection of the particular protectiveagent for a particular metal is known in the art and will not bedetailed here. Suitable metal protective agents for practising thepresent invention include sodium carbonate or sodium metasilicate. Themetal protective agent is present in the amount of about 0.5 to 2.0percent by weight of the composition.

The optimum operating temperature of the present composition to provideeffective contaminant removal within a short period of time is withinthe range of 51.7 to 71.1° C. (125° to 160° F.). Within this range, fluxcontaminants may be removed in 1 to 3 minutes. A preferred temperaturefor use of the present composition is about 60° C. (140° F.). Theincreased temperature promotes the reaction, improves penetration of thecomposition by softening gummy contaminants, and keeps particulates insuspension so that they do not deposit on the cleaned substrates. Inaddition, the elevated temperature optimizes the effectiveness of theultraviolet radiation for cleaving bonds in organic materials, asdescribed below.

The contaminants which can be removed in accordance with the presentinvention comprise organic materials which include, but are not limitedto the residues left by commonly used flux materials. These fluxresidues comprise oils, resins, and other organic materials. Since thepresent composition and method are effective for cleaving carbonto-carbon bonds, they may be used for the removal of other organiccompounds as well. Such materials include, but are not limited to, oil,grease, lubricants, photoresist, adhesive residues, plasticizers, ordyes.

The solid substrate from which contaminants can be removed in accordancewith the present invention may comprise any material which is notadversely affected by the peroxide or alkaline components of the presentcomposition. Such materials include, but are not limited to,polyimide/polyamide laminates and epoxy/glass laminates which are usedin printed wiring boards, silicon which is used in electronic devices,and anodized aluminum or polyimide which are used in cables andconnectors. The solid substrate may have a simple or complexconfiguration and may include interstitial spaces which are difficult toclean by known methods. The substrate may be in the form of a continuouslayer or in the form of discrete particles.

In accordance with an alternative embodiment of the present invention,ultraviolet radiation is used to enhance the cleaning process. Whenhydrogen peroxide is exposed to ultraviolet radiation having awavelength within the range of 184 to 300 nanometers (nm), preferablyabout 253 nm, such as from a xenon flash lamp, the hydrogen peroxide isdissociated to form a hydroxyl radical (OH⁻) which is very reactive.This hydroxyl radical then cleaves the carbon-to-carbon bonds in theorganic contaminant material, forming carbon dioxide and water. Inaddition, the ultraviolet radiation itself also cleaves thecarbon-to-carbon bonds in the organic contaminant materials, adding evenfurther effectiveness to the cleaning process. The source of ultravioletradiation, such as a xenon flash lamp or a mercury vapor lamp, islocated external to the chamber containing the substrate to be cleanedand the present composition, and the radiation is directed into thecleaning chamber. For this purpose, a quartz window is provided on onesurface of the cleaning chamber to permit transmission of theultraviolet radiation. Optionally, a focusing barrel may be providedbetween the quartz window and the radiation source to improve theefficiency of the ultraviolet radiation transmission. Such methods forintroducing radiation into reaction chambers are well known.

The composition of the present invention may be used in essentially thesame manner as known defluxing agents are used, namely in a spray orsoak operation. If used in a soak operation, it is desirable to keep thepresent composition agitated, by air or mechanical or ultrasonic means.As an added feature in the present method, after the substrate has beentreated with the present composition, residual alkaline materials(alkali or alkali salts) remaining on the substrate are neutralized. Apreferred neutralization agent comprises hot carbonated water, which isapplied at 51.7° to 71.1° C. (125° to 160° F.). The carbonated water maybe formed by bubbling carbon dioxide into deionized water. The use ofthe carbonated water rinse keeps the rinse water below a pH of 9.5,which is the regulated limit for disposal in a sewer. After theneutralization step, the substrate is rinsed in hot water at 51.7° to71.1° C. (125° to 160° F.), preferably with air agitation, and thendried, preferably with hot air. In an exemplary batch processingtechnique, in which contaminated substrates are loaded into a chamberand treated, in turn, with the present composition, neutralizing agent,water, and hot air, the following approximate processing times maytypically be used:

    ______________________________________                                        Step                   Time                                                   ______________________________________                                        a.    Exposure of contaminated substrate                                                                 3 minutes                                                to present composition                                                  b.    Neutralization       3 minutes                                          c.    Hot water rinse      4 minutes                                          d.    Hot air drying       5 minutes                                          ______________________________________                                         Thus, the present process can be completed within 15 minutes.

Alternatively, the present process may be practised as an "in line"process. The contaminated substrates are loaded into a rack and the rackis lowered in sequence into a series of chambers containing,respectively, the present composition, the neutralizing agent, anddeionized water sprayers. As yet another alternative, this in lineprocessing may be accomplished by a conveyorized system.

After the substrate has been treated with the present composition, thesubstrate is removed from the chamber containing the composition. Thestarting composition is then re established by the addition of hydrogenperoxide and, if necessary, the other components of the composition.Thus, the bulk cleaning solvent remains in place and does not requiredisposal. Rather, the composition is regenerated in situ by the additionof component materials as required.

As an added feature of the present invention, the present compositionremoves metallic oxides from the substrate and thus serves also as asolder brightener. This eliminates the need for a secondary process,such as treatment with fluoroboric acid.

Examples of practise of the present invention are as follows.

EXAMPLE 1

This example illustrates the use of the composition and method of thepresent invention to remove solder flux from printed wiring boards.

It should be noted that while the present composition is non toxic,general safety precautions, such as the use of goggles and protectiveclothing, are necessary.

The test samples comprised polyimide/polyamide laminate printed wiringboards containing copper metallization and having been exposed to Alpha611, a solder flux obtained from Alpha Metals Company of Alpharetta,Ga., and comprising a mildly activated rosin flux. The composition ofthe present invention comprised: approximately 3.0 percent by weighthydrogen peroxide; approximately 0.2 percent by weight sodium hydroxide;approximately 0.1 percent by weight sodium 2-ethylhexyl sulfate as thewetting agent; approximately 0.5 percent by weight sodium metasilicateas the metal protective agent; and the balance being purified water. Thetemperature of the composition was maintained at 60° C. (140° F.). Thesamples were treated in a batch process as previously described hereinusing the previously described processing steps and times. The presentcomposition was nearly transparent prior to introduction offlux-contaminated materials, with only trace amounts of gas liberationvisible. When the contaminated material was introduced, vigorous gasformation on the flux residue was observed. This action continued as theflux was emulsified and dissolved into solution. Continued gas formationwas visible as the dissolved flux residues continued to be oxidized.Completion of the decomposition was indicated by slow gas evolution andchange in fluid color. Surface foaming was minimal. Complete removal ofthe flux residues was accomplished in an average of less than twominutes. The soldered joints were clean and free of oxides, i.e., brightin appearance.

By contrast, it was found that alkaline solutions alone did noteffectively separate the bulk of the flux residues from the substrate.In addition, alkaline solutions with wetting agents, but withouthydrogen peroxide, required in excess of 10 minutes to separate fluxresidues from the substrate. Even after separation, the flux residuesdid not disperse well.

EXAMPLE 2

This example illustrates the use of the composition and method of thepresent invention to remove solder flux from cable connectors.

A soft anodized aluminum connector was treated with the composition ofthe present invention as described in Example I except using atemperature of 160° F. (71° C.) and 60-second rinse times for both theneutralization and final rinse steps. There was no visually detectabledegradation of the anodized aluminum by exposure to the presentcomposition.

In another experiment, a cable made of Kapton (a trademark of E.I. duPont de Nemours for a polyimide) was contaminated with Kester 197, amildly activated rosin based flux obtained from Litton-Kester Company ofChicago, Ill. The Kapton cable was treated in accordance with thepresent invention as described above. Complete removal of the flux wasaccomplished as determined by visual inspection under 25X magnification.

EXAMPLE 3

This example illustrates the use of the embodiment of the method of thepresent invention in which ultraviolet radiation is used to enhance thecontaminant removal.

The composition and method described in Example 1 are used except thatthe composition and the substrate containing contaminants are exposed toradiation from a xenon flash lamp as previously described herein. Thecleaning process proceeds generally as described in Example 1, exceptthat complete flux removal is accomplished more rapidly.

As previously discussed, the organic contaminants are oxidized by thepresent composition to form carbon dioxide, nitrogen, and water. Thecarbon dioxide and nitrogen are non-hazardous and may be exhausted intothe atmosphere. The water by-product contains no noxious material andmay be disposed of in a sewer without further treatment or may be reused. The hydrogen peroxide in the present composition is decomposedduring the cleaning operation into atomic oxygen or hydroxyl radicalsand water and the former react with the contaminants to form theabove-noted products. Insoluble precipitates, such as cellulose gums,may also be formed as by products of the bond cleaving of the presentcomposition, and may be readily removed by filtration. Thus, no toxic orhazardous materials are formed as a result of the present method.Consequently, the present process obviates the need for the expensivesolvent emission control and waste management procedures required whenusing prior art halogenated solvents. In addition, environmentalpollution and exposure of workers to hazardous materials are alsoavoided by the present composition and method.

While the previous discussion has focused on the use of the presentcomposition and method to remove flux residues from solid substrates, itis not intended to limit the present invention to this particularcontaminant. Rather, it is intended to include within the scope of thepresent invention the removal of any organic material from a given solidsubstrate.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited to the specificembodiments as illustrated herein, but is only limited by the followingclaims.

What is claimed is:
 1. A method for removing organic flux residue from asolid substrate while simultaneously avoiding negative environmentalimpact, said method comprising:(a) providing a compositioncomprising:(1) hydrogen peroxide in the amount of about 3 to 5 percentby weight of said composition; (2) an alkali metal hydroxide insufficient amount to provide a pH of at least 10.5 in said composition;(3) a chosen wetting agent in the amount of about 0.1 to 0.3 percent byweight of said composition, said wetting agent being unreactive withsaid hydrogen peroxide and said alkali metal hydroxide; and (4) purifiedwater as the balance of said composition; and (b) exposing said solidsubstrate having said organic flux residue thereon to said compositionat a temperature within the range of 51.7° to 71.1° C. or 125° to 160°F., whereby said organic flux residue is removed from said substrate andconverted into non-toxic and non-hazardous products.
 2. The method ofclaim 1 further comprising after said exposing said substrate to saidcomposition, neutralizing residual said alkali metal hydroxide thereformaterials on said substrate, rinsing said substrate with water, anddrying said substrate.
 3. The method of claim 2 wherein saidneutralizing comprises exposing said substances to hot carbonated water.4. The method of claim 1 wherein said exposing in step "b" furthercomprises exposing said composition and said flux residue on saidsubstrate to ultraviolet radiation having a wavelength within the rangeof 184 to 300 nanometers.
 5. The method of claim 4 wherein saidradiation has a wavelength of about 253 nanometers.
 6. The method ofclaim 1 wherein said substrate comprises a metal and said compositionfurther comprises a metal protective agent in the amount of about 0.5 to2.0 percent by weight of said composition, said metal protective agentbeing unreactive with said hydrogen peroxide and said alkali metalhydroxide to thereby protect said metal substrate from attack by saidhydrogen peroxide and said alkali metal hydroxide.
 7. The method ofclaim 6, further comprising after said exposing said substrate to saidcomposition, neutralizing residual said alkali metal hydroxide thereformaterials on said substrate, rinsing said substrate with water, anddrying said substrate.
 8. The method of claim 7 wherein saidneutralizing comprises exposing said substrate to hot carbonated water.9. The method of claim 6 further comprising exposing said compositionand said flux residue on said substrate to ultraviolet radiation havinga wavelength within the range of 184 to 300 nanometers.
 10. The methodof claim 9 wherein said radiation has a wavelength of about 253nanometers.